<meta charset="UTF-8">
<?php
if(isset($_POST['button'])):
    $color=$_POST['color'];
    switch($color):
        case '1':
            $color='#FF0000';
            break;
        case '2':
            $color='#009900';
            break;
        case '3':
            $color='#0000FF';
            break;
    endswitch;
    ?>
    <script type="text/javascript">
        window.onload=function(){
            shi.style.color="<?php echo $color?>";
        }
    </script>
    <?php
endif;
?>
<div id="shi" class="">
    锄禾日当午，<br />
    汗滴禾下土。<br />
    谁知盘中餐，<br />
    粒粒皆辛苦。<br />
</div>
<form method="post" action="">
    <select name="color">
        <option value="">---请选择颜色---</option>
        <option value="1">红色</option>
        <option value="2">绿色</option>
        <option value="3">蓝色</option>
    </select> <br />
    <input type="submit" name="button" value="更改颜色">
</form>